This invention relates to improvements in medical separatory devices, such as hollow fiber artificial kidneys, utilizing the type of hollow fiber bundle disclosed in Mahon U.S. Pat. No. 3,228,876.
Artificial kidneys employing a single Mahon-type bundle comprising thousands of semi-permeable cellulose or cellulose acetate hollow fibers, after commercial introduction in the United States by Cordis Dow Corp. in the late 1960's have experienced worldwide acceptance and the proportion of such kidneys in use has continuously increased since that time.
Commercial versions of such artificial kidneys having the general configuration of a tube-and-shell heat exchanger similar to that shown in U.S. Pat. No. 3,228,877 typically comprise a shell of about 4" to 10" in length and 11/2" to 4" in diameter and usually contain more than 5,000 and less than 25,000 individual hollow fibers. The shell is divided into two blood chambers which are spaced apart by an intervening dialysate chamber within which the hollow semi-permeable fibers, carrying blood in their interiors, are continuously bathed on their exterior surfaces by a dialysate solution. The fibers are secured to each other in a solidified resin tubesheet and the tubesheet, on each end of the fibers, serves to seal the blood chamber from the intervening dialysate chamber. The great majority of this type of artificial kidney have employed a cylindrical dialysate chamber integrally attached to end blood chambers, each having a larger diameter than the dialysate chamber, such as illustrated in U.S. Pat. Nos. 3,228,876; 3,228,877; and most particularly in FIG. 4 of U.S. Pat. No. 3,882,024. The tubesheets are typically formed by centrifugally casting a castable synthetic resin around the fibers while those fibers are positioned within the shell in such a manner that the solidified tubesheet assumes the cylindrical cross section of the enlarged blood chamber portion of the shell. Widely used fiber potting, or centrifugal casting, processes of this type are disclosed in U.S. Pat. Nos. 3,442,002 and 3,492,698. Another usable method is shown in U.S. Pat. No. 3,755,024.
An inherent advantage of centrifugally casting the supporting tubesheet directly in the kidney shell is that the tubesheet automatically serves as the separating barrier between the dialysate and blood chambers. Moreover, this separating barrier concurrently forms a fluid tight seal between the dialysate and blood chambers and this seal results from the contact between the surface of the inner wall of the shell and the surface of the outer wall of the tubesheet at its rim. Ideally, the desired seal occurs due to adhesion between the selected castable resin and the shell as the tubesheet resin solidifies in that shell. In commercial manufacture by this method, multi-thousands of artificial kidneys have been made and used satisfactorily. However, care must be taken to select materials for the shell, the tubesheet and the hollow fibers which compatibly bind the fibers together in the tubesheet and concurrently maintain the necessary fluid tight seal between the shell and tubesheet under the variable temperature and pressure conditions encountered in transporting and using the kidneys for their intended purpose. Extensive research has been directed to the problems inherent in maintaining these vitally important seals. Epoxy type tubesheet compositions have been explored and identified in U.S. Pat. Nos. 3,619,459 and 3,703,962. Suitable permselective fibers are disclosed in 3,423,491, while polyurethane tubesheet compositions are disclosed in U.S. Pat. Nos. 3,362,921; 3,643,805; 3,708,071; and 3,962,094.
Notwithstanding the above research efforts, manufacturing difficulties remain due to the exacting tolerance requirements on the peripheral dimensions and on the uniformity of those dimensions on the individual elements to insure and maintain satisfactory sealing to meet manufacturing specifications or the actual conditions encountered during clinical use. One problem that has persisted results from the tendency of the hollow fibers to expand under use conditions, that is, when wet with blood internally, surrounded by tubesheet resin externally, and wet with dialysate on the external surfaces of the fibers immediately adjacent to the inner end of the tubesheet surface. It has been observed that such expansion may be sufficient to cause rupturing of the seal between the shell and the tubesheet rim. Similarly, rupture between the shell and tubesheet may result from unequal shrinkage of the tubesheet resin away from the inside shell wall during solidification, or a partial rupture may occur during testing or in use where the fibers are not centered in the tubesheet relative to the shell.
Another recurring problem encountered from time to time in commercial manufacture using centrifugal potting that produces a tubesheet containing off center fibers is that of clotting in stagnant zones on the tubesheet surface which exposes the open fiber ends to the blood pool in the blood chamber. The blood chamber is usually formed by a cup-shaped member which is sealed against the planar surface of the tubesheet by a conventional circular O-ring such as shown in FIG. 4 of U.S. Pat. No. 3,882,024. Where the hollow fibers extending through the tubesheet are off center a stagnant zone occurs in the out of round portion, or arcuate segment, encompassed by the circular O-ring. In that segment, the tubesheet surface is solid and no open ends of fibers are available to accept the blood and stagnation of the blood in that location over a portion of the hemodialysis treatment may cause clotting. In addition to the undesirable loss of blood to the patient, such clots are difficult to remove, or cleanse during backwashing in preparation for possible reuse of the kidney and, where not removed require discarding of the kidney.
The primary object of this invention is to provide an improved separatory device which overcomes the above described problem of failure in the tubesheet-shell seal.
A second important objective of this invention is to provide an improved artificial kidney device which eliminates the blood clotting problem caused by off center fibers in the tubesheet.
Another objective of this invention is to provide an improved method of centrifugally casting semi-permeable hollow fibers into a castable resin while those fibers are positioned in the artificial kidney shell which insures that the fibers remain centered in the tubesheet and extend to the peripheral surface of that tubesheet at its transverse planar surface which exposes the open fiber ends to the interior of the blood chamber.